Baby zombies cannot be turned into normal zombies using NBTTags. This can be tested using a modification or using the command /data. This is caused by a little bug in the code that I've already located.

How to reproduce:

1. Spawn a zombie.
2. Turn it into a baby zombie using the command /execute as @e[type=minecraft:zombie] run data merge entity @s {IsBaby:1}
3. Try to turn it into a normal zombie using /execute as @e[type=minecraft:zombie] run data merge entity @s {IsBaby:0}

 The error in the code: (Class EntityZombie)